Slag Conditioner for Electric Arc Furnace Steel Production

ABSTRACT

A slag conditioner for electric arc furnace steel production comprising a carbonate-containing material with the balance being a reducing agent that comprises a reducing element that is easily oxidized in an exothermic reaction. The slag conditioner may further include carbonaceous material and/or an MgO-containing material. The slag conditioners may be in particulate, pellet, or briquette form. Also, a method of conditioning the slag in an electric arc furnace where steel in being produced, the method comprising introducing the particulate or pellet slag conditioners into the slag or into an interface between the slag and the molten metal or charging the briquette slag conditioners into the top of the furnace.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application claims priority to U.S. Provisional Patent Application No. 62/519,417, filed Jun. 14, 2017 entitled “Slag Conditioner for Electric Arc Furnace Steel Production”, the disclosure of which is hereby incorporated in its entirety by reference.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to compositions useful for the making of steel, and more particularly, to the composition of a slag conditioner, a method for producing such a slag conditioner, and a method of conditioning slag in an electric arc furnace using such slag conditioner.

Description of Related Art

Prior to 1960, silica brick was primarily used in the refractory linings of steelmaking furnaces such as electric arc furnaces (“EAFs”). The silica brick was compatible with the acidic silicon dioxide (SiO₂) produced in the slag of the steelmaking process, resulting in extended life of the refractory lining. As demand for steel grew, however, refractory linings became unable to keep pace with the increased use of the furnaces using increased operating temperatures, and the lifespan of refractory linings were significantly shortened. Such shortcomings increased the amount of downtime for repairs and maintenance and increased the costs of producing steel.

As the need for improved refractory linings became more pressing, a change from silicon dioxide linings to more basic linings based on magnesium oxide (MgO) and calcium oxide (CaO) grew in popularity. These refractory linings were principally composed of burned dolomite and/or dead burned magnesite. Because these new linings were substantially more basic than the previously utilized silica linings, the composition of slag had to be changed.

Molten slag is ionic in nature, consisting both of cations and anions. The principal anion in slag is silicate as contributed by impurities in the scrap, and the basic building block of this silicate is the silicate tetrahedron (SiO44-). The addition to the slag of, among other metal oxides, CaO and MgO, results in a breakdown of the tetrahedron structure forming liquid silicate compounds. The addition of CaO to slag is important for a number of reasons. First, it makes the slag more basic for improved interaction with the refractory lining and increasing lining durability. Second, CaO improves the ability of the slag to remove impurities from the liquid steel. It was noted, however, that the weight percent ratio of CaO to SiO₂ (C/S weight ratio) present in the slag at the conclusion of steelmaking impacts the level of MgO needed for the process because MgO is soluble in calcium silicate liquid slags, which also contain other oxides such as FeO and Al₂O₃. A CaO/SiO₂ molar ratio (C/S mol ratio) of greater than 2-to-1 requires a significant percentage of MgO to be present in the slag. If the required percentage is not present, the process leaches the additional quantity of MgO from the refractory lining of the furnace, resulting in decreased lining durability. A C/S mol ratio of less than 2-to-1 also dissolves MgO, but at an even higher percentage and is to some extent dependent on the FeO content. Thus, it became common practice to include higher amounts of MgO in the slag.

To satisfy this demand for increased MgO content in slag, steelmakers began adding higher levels of burned dolomite or a mixture of burned dolomite and burned limestone to the slag. This resulted in increased refractory lining lifespan. However, maintenance was still needed on a frequent basis, resulting in increased downtime for steelmaking furnaces.

To combat this downtime, steelmakers began to experiment further with slag, resulting in new compositions and a foaming slag. Adjusting the C/S weight ratio to between 1.7 and 2.1 increased the viscosity of the slag while also increasing the amount of MgO that is dissolved. Increasing the MgO concentration of the slag also made the slag more viscous. It was known, based on experiences in basic oxygen furnaces (“BOFs”), that increased viscosity increases the slag that splashes onto the refractory walls. This splashing effect protects the walls of the furnace from excessive wear and reduces downtime for the steelmaking furnaces.

Just increasing the viscosity was not enough in EAFs because special requirements exist with regard to slag in EAFs. For instance, slag splashed onto the walls of the furnace is necessary to protect the lining from electrical arc radiation. Additionally, the use of direct reduced iron in the steelmaking process and the use of liquids with low-melting temperature silicates, results in high MgO solubility and a need for increased MgO concentrations in the slag to prevent leeching of MgO from the refractory lining.

The amount of slag splashed onto the refractory lining of an EAF can be increased by the injection of oxygen gas into the steelmaking chamber. This gas and available FeO in the slag reacts with carbon present in coal or coke to form carbon monoxide (CO) and carbon dioxide (CO₂). The production of these gases forms bubbles in the slag, increasing the slag volume and creating a “foamy” slag which helps coat the electrodes and the refractory lining of the furnace walls. However, during the production of stainless steel, little or no oxygen is introduced into the furnace, iron is not oxidized, and iron oxide (FeO) in the slag is low, typically, less than 2%. Therefore, there is no need for carbon injection to reduce the FeO as described above. Without the carbon injection, no carbon monoxide or carbon dioxide gases are produced to foam the slag to protect the refractory lining of the furnace. As a result, the refractory lining of the furnace must be replaced frequently, typically every 1-3 months.

There is, therefore, a need for a slag conditioner that foams the slag and/or increases the viscosity of the slag to better protect the furnace refractory lining regardless of the type of steel that is being produced.

SUMMARY OF THE INVENTION

The present invention is directed to a slag conditioner for electric arc furnace steel production comprising 50-90 wt. % of a carbonate-containing material with the balance being a reducing agent that comprises a reducing element that is easily oxidized in an exothermic reaction, wherein the weight ratio of CO₃ to the reducing element (CO₃:reducing element) for the slag conditioner is 3-20. The slag conditioner may further include 5-25 wt. % of an MgO-containing material such that the CO₃ to MgO (CO₃:MgO) weight ratio for the slag conditioner is 1-15.

The present invention is also directed to a slag conditioner for electric arc furnace steel production comprising a carbonate-containing material, a carbonaceous material, and a reducing agent that comprises a reducing element that is easily oxidized in an exothermic reaction. The slag conditioner comprises 10-40 wt. % of the carbonate-containing material and 8-87 wt. % of the carbonaceous material with the balance being the reducing agent. The weight ratio of CO₃ to the reducing element (CO₃:reducing element) may be 3-20, and the weight ratio of CO₃ to the carbon provided by the carbonaceous material (CO₃:C) may be 0.1-5. The slag conditioner may further include 5-25 wt. % of an MgO-containing material such that the CO₃ to MgO (CO₃:MgO) weight ratio for the slag conditioner is 0.1-10.

The carbonate-containing material may be one or more material selected from the group consisting of dolomite and limestone, and the reducing element may be selected from the group consisting of silicon and aluminum. At least 50% of the MgO in the MgO-containing material may be periclase, and the MgO-containing material may be one or more material selected from the group consisting of dead burned dolomite, dead burned magnesite, dead burned brucite, fused dolomite, fused magnesite, fused brucite, recycled MgO-containing slags, and pre-fired MgO-containing refractories including recycled magnesium oxide-carbon refractory bricks, recycled magnesium oxide-spinel refractory bricks, recycled MgO bricks, recycled magnesia-alumina-carbon bricks, recycled MgO-based tundish lining material, and recycled dead burned dolomite brick.

The slag conditioner may be a particulate comprising particles. The particles may be 6 mm or less. Alternatively, the slag conditioner may be pellet form or may be a briquette, and the pellets or briquette may further include 1-14 wt. % of a binder.

The present invention is also directed to a method of conditioning the slag in an electric arc furnace where steel is being produced, the method comprising introducing the particulate or pellet slag conditioners described above into the slag or into an interface between the slag and the molten metal or charging the briquette slag conditioner described above into the top of the furnace.

DESCRIPTION OF THE INVENTION

As used herein, unless otherwise expressly specified, all numbers such as those expressing values, ranges, amounts or percentages may be read as if prefaced by the word “about”, even if the term does not expressly appear. Any numerical range recited herein is intended to include all sub-ranges subsumed therein. For example, a range of “1 to 10” is intended to include any and all sub-ranges between and including the recited minimum value of 1 and the recited maximum value of 10, that is, all subranges beginning with a minimum value equal to or greater than 1 and ending with a maximum value equal to or less than 10, and all subranges in between, e.g., 1 to 6.3, or 5.5 to 10, or 2.7 to 6.1. Plural encompasses singular and vice versa. When ranges are given, any endpoints of those ranges and/or numbers within those ranges can be combined with the scope of the present invention. “Including”, “such as”, “for example” and like terms means “including/such as/for example but not limited to”. All percentages are in terms of weight percent.

The present invention is directed to a slag conditioner comprising a carbonate-containing material, a reducing agent and, optionally, a magnesium oxide-containing material (MgO) and/or carbonaceous material that may be introduced into the slag layer or the slag/metal interface of an electric arc furnace (EAF) or charged into the top of an EAF. The slag conditioner may also include a binder and/or other compatible fillers.

Slag Conditioner with Carbonate-containing Material and a Reducing Agent

The slag conditioner may comprise at least 50 wt. % of a carbonate-containing material and up to 90 wt. % of a carbonate-containing material, for example, 50-90 wt. % of a carbonate-containing material, 60-90 wt. % of a carbonate-containing material, or 70-90 wt. % of a carbonate-containing material, with the balance being a material containing an element that is easily oxidized in an exothermic reaction, for example, silicon or aluminum. The carbonate-containing material may be one or more selected from the group including, but not limited to, dolomite, magnesite, limestone, and dolomitic limestone. Dolomite as used herein is defined as calcium magnesium carbonate (CaMg(CO₃)₂) that has not been calcined or burned. Magnesite as used herein is defined as magnesium carbonate (MgCO₃) that has not been calcined or burned. Limestone as used herein is defined as calcium carbonate (CaCO₃). Dolomitic limestone as used herein is defined as up to 90 wt. % calcium carbonate (CaCO₃) in combination with dolomite (CaMg(CO₃)₂). The carbonate-containing material may comprise particles that are small enough to be incorporated into the slag, but not so small that they are swept up by the furnace draft into the exhaust system. For example, the particles, when screened may be 12 mm or less in diameter, 10 mm or less in diameter, 8 mm or less, 6 mm or less, or 3 mm or less in diameter, i.e., the particles, for example, pass through a mesh having 12 mm, 10 mm, 8 mm, 6 mm, or 3 mm openings, respectively. Very fine particles, 63 μm (230 mesh) may be limited to 15% or less except for carbonate-containing materials that are to be a component of a pelletized product. Materials used for pellet making have no practical limit on particle size since fine particles will be agglomerated with binders in the mixing process.

The reducing agent comprises an element that is easily oxidized in an exothermic reaction and have a free energy of oxide formation that is less than the free energy of oxide formation for iron at the temperature at which the slag conditioner is introduced into the EAF. Such elements are, for example, silicon, aluminum, manganese, vanadium, and titanium, hereinafter, the reducing element. The reducing agent may be in the form of metal grains or fines, a carbide or a non-toxic salt. The reducing agent may be one or more selected from the group including, but not limited to, silicon metal fines, silicon carbide (SiC), and/or ferrosilicon. The reducing agent may comprise particles that, when screened, are 8 mm or less, for example, 6 mm or less, 3 mm or less (˜7 mesh), 1.5 mm or less (˜14 mesh), 0.5 mm or less (35 mesh), or 0.25 mm or less (60 mesh), i.e., the particles pass through a mesh having openings of 6 mm, 3 mm, 1.5 mm, 0.5 mm, or 0.25 mm, respectively. The reducing agent may comprise particles that are similar in size to the particles of carbonate-containing material, for example, 6 mm or less, if the slag conditioner is to be incorporated into the slag without being pelletized. If the slag conditioner is to be pelletized, the reducing agent may comprise particles that are similar in size to the particles of the CO₃-containing material or are smaller than the particles of the CO₃-containing material.

The carbonate-containing material and the reducing agent are contained in amounts such that the carbonate to reducing element (CO₃:reducing element) weight ratio of the slag conditioner is at least 3 and up to 20, for example, 3-20, 3-15, 3-10, or 4-9.

A first example of the slag conditioner comprises 85 wt. % dolomite which contains 65 wt. % carbonate and 15 wt. % silicon carbide which 70 wt. % contains reducing element (Si). Thus, the slag conditioner contains 55.3 wt. % carbonate and 10.5 wt. % reducing element (Si) for a CO₃:reducing element ratio of 55.3:10.5 or 5.3.

A second example of the slag conditioner comprises 90 wt. % dolomite which contains 65 wt. % carbonate and 10 wt. % silicon carbide which contains 70 wt. % reducing element (Si). Thus, the slag conditioner contains 58.5 wt. % carbonate and 7.0 wt. % reducing element (Si) for a CO₃:reducing element ratio of 58.5:7.0 or 8.4

A third example of the slag conditioner comprises 85 wt. % magnesite containing 71 wt. % carbonate, and 18 wt. % silicon carbide which contains 70 wt. % reducing element (Si). Thus, the slag conditioner contains 60.4 wt. % carbonate and 12.6 wt. % reducing element (Si) for a CO₃ :reducing element ratio of 60.4:12.6 or 4.8.

A fourth example of the slag conditioner comprises 90 wt. % magnesite containing 71 wt. % carbonate, and 12 wt. % silicon carbide which contains 70 wt. % reducing element (Si). Thus, the slag conditioner contains 63.9 wt. % carbonate and 8.4% reducing element (Si) for a CO₃:reducing element ratio of 63.9:8.4 or 7.6.

This slag conditioner may be best suited for stainless steel production. During the production of stainless steel, since little or no oxygen is introduced into the furnace, iron is not oxidized and iron oxide (FeO) in the slag is low, typically, less than 2%. Therefore, there is no need for a carbon addition to reduce the FeO as is done in carbon steel production.

Slag Conditioner with Carbonate-Containing Material, a Reducing Agent, and an MgO-Containing Material

The slag conditioner described above which contains a carbonate-containing material and a reducing agent may also contain an MgO-containing material.

The slag conditioner may comprise at least 5 wt. % of an MgO-containing material and up to 25 wt. % of a MgO-containing material, for example, 5-25 wt. % of an MgO-containing material, 5-20 wt. % of an MgO-containing material, or 10-20 wt. % of an MgO-containing material.

The MgO-containing material may be any material where at least 50% of the contained MgO is periclase. As used herein, periclase is defined as the cubic crystalline, non-reactive or less reactive form of MgO that can be identified using X-ray diffraction and remains in solid form when introduced into a saturated or partially saturated slag. The MgO-containing material may be one or more material selected from the group including, but not limited to, dead burned dolomite, dead burned magnesite, dead burned brucite, fused dolomite, fused magnesite, fused brucite, recycled MgO-containing slags, and pre-fired MgO-containing refractories including recycled magnesium oxide-carbon refractory bricks, recycled magnesium oxide-spinel refractory bricks, recycled MgO bricks, recycled magnesia-alumina-carbon bricks, recycled MgO-based tundish lining material, and recycled dead burned dolomite brick. The pre-fired MgO-containing refractories may contain dead burned dolomite, dead burned magnesite, fused MgO, and/or fused dolomite.

Dead burned dolomite as used herein is defined as dolomite, calcium magnesium carbonate (CaMg(CO₃)₂), that has been calcined or burned at greater than 1500° C., thereby forming CaO and MgO as periclase. Dead burned magnesite as used herein is defined as magnesite, magnesium carbonate (MgCO₃), that has been calcined or burned at 1500-2000° C., thereby forming MgO as periclase. Dead burned brucite as used herein is defined as brucite, magnesium hydroxide (MgOH₂), that has been calcined or burned at 1500-2000° C., thereby forming MgO as periclase. Fused dolomite as used herein is defined as dolomite, calcium magnesium carbonate (CaMg(CO₃)₂), that has been calcined or burned at >2750° C., thereby forming MgO as periclase. Fused magnesite as used herein is defined as magnesite, magnesium carbonate (MgCO₃), that has been calcined or burned at >2750° C., thereby forming MgO as periclase. Fused brucite as used herein is defined as brucite, magnesium hydroxide (MgOH₂), that has been calcined or burned at >2750° C., thereby forming MgO as periclase.

The MgO-containing material may comprise particles that, when screened, are 8 mm or less, for example, 6 mm or less, 3 mm or less (˜7 mesh), 1.5 mm or less (˜14 mesh), 0.5 mm or less (35 mesh), or 0.25 mm or less (60 mesh), i.e., the particles pass through a mesh having openings of 6 mm, 3 mm, 1.5 mm, 0.5 mm, or 0.25 mm, respectively. The MgO-containing material may comprise particles that are similar in size to the particles of the carbonate-containing material, for example, 6 mm or less or 3 mm or less, if the slag conditioner is to be incorporated into the slag without being pelletized. If the slag conditioner is to be pelletized, the MgO-containing material may comprise particles that are similar in size to the particles of the carbonate-containing material or are smaller than the particles of the carbonate-containing material.

The carbonate-containing material and the MgO-containing material are contained in amounts such that the carbonate to MgO (CO₃:MgO) weight ratio of the slag conditioner is at least 1 and up to 15, for example, 1-15, 2-15, or 2-13.

A first example of the slag conditioner comprises 70 wt. % dolomite which contains 65 wt. % carbonate, 12 wt. % silicon carbide which contains 70 wt. % reducing element (Si), and 18 wt. % of an MgO-containing material which contains 80 wt. % MgO. Thus, the slag conditioner contains 45.5 wt. % carbonate, 8.4 wt. % reducing element (Si), and 14.4 wt. % MgO for a CO₃:reducing element ratio of 55.3:10.5 or 5.3 and a CO₃:MgO ratio of 45.5:14.4 or 3.2.

A second example of the slag conditioner comprises 72 wt. % dolomite which contains 65 wt. % carbonate, 10 wt. % silicon carbide which contains 70 wt. % reducing element (Si), and 18 wt. % of an MgO-containing material which contains 80 wt. % MgO. Thus, the slag conditioner contains 46.8 wt. % carbonate, 7.0 wt. % reducing element (Si), and 14.4 wt. % MgO for a CO₃:reducing element ratio of 46.8:7.0 or 6.7 and a CO₃:MgO ratio of 46.8:14.4 or 3.3.

A third example of the slag conditioner comprises 70 wt. % magnesite which contains 71 wt. % carbonate, 12 wt. % silicon carbide which contains 70 wt. % reducing element (Si), and 18 wt. % of an MgO-containing material which contains 80 wt. % MgO. Thus, the slag conditioner contains 49.7 wt. % carbonate, 8.4 wt. % reducing element (Si), and 14.4 wt. % MgO for a CO₃:reducing element ratio of 49.7:8.4 or 5.9 and a CO₃:MgO ratio of 49.7:14.4 or 3.5.

A fourth example of the slag conditioner comprises 72 wt. % magnesite which contains 71 wt. % carbonate, 10 wt. % silicon carbide which contains 70 wt. % reducing element (Si), and 18 wt. % of an MgO-containing material which contains 80 wt. % MgO. Thus, the slag conditioner contains 5.10 wt. % carbonate, 7.0 wt. % reducing element (Si), and 14.4 wt. % MgO for a CO₃:reducing element ratio of 51.1:7.0 or 7.3 and a CO₃:MgO ratio of 51.1:14.4 or 3.6.

This slag conditioner may be best suited for stainless steel production. During the production of stainless steel, since little or no oxygen is introduced into the furnace, iron is not oxidized and iron oxide (FeO) in the slag is low, typically, less than 2%. Therefore, there is no need for a carbon addition to reduce the FeO as is done in carbon steel production.

Slag Conditioner with Carbonate-Containing Material, a Reducing Agent, and a Carbonaceous Material

The slag conditioner described above which contains a carbonate-containing material and a reducing agent may also contain a carbonaceous material.

The slag conditioner may comprise at least 8 wt. % carbonaceous material and up to 87 wt. % carbonaceous material, for example, 8-87 wt. % carbonaceous material, 20-80 wt. % carbonaceous material, or 50-70 wt. % carbonaceous material.

The carbonaceous material may be one or more material selected from anthracite coal, semi-anthracite coal, bituminous coal, natural graphite, synthetic graphite, petroleum coke, metallurgical coke, spent EAF electrodes, spent carbon anodes, and carbon black. The carbonaceous material may contain up to 15 wt. % moisture, for example, 5-12 wt. % moisture, and at least 50 wt. % carbon, for example, at least 70 wt. % carbon or 75-99 wt. % carbon. For example, metallurgical coke may have 5-6% moisture and coal may have 8-12% moisture. Carbonaceous materials having moisture contents of 2 wt. % or greater may be pre-dried or may be used without drying when making slag conditioner in the form of pellets or briquettes. If the carbonaceous material is not dried, the pellets or briquettes may be dried after mixing and forming. The carbonaceous material may comprise particles of sufficiently small size to be transportable through a pneumatic pipe injection system into the furnace, small enough to be incorporated into the slag and not into the steel, and large enough that, when injected into the steelmaking furnace, the particles are not deflected by furnace draft. For example, the particles, when screened may be 12 mm or less in diameter, 10 mm or less in diameter, or 3 mm or less in diameter, i.e., the particles, for example, pass through a mesh having 12 mm, 10 mm, or 3 mm openings, respectively. For example, the carbonaceous material may be #4 anthracite coal (1.2-2.4 mm) or #5 anthracite coal (0.15-1.2 mm), or may be petroleum coke particles that are less than 12 mm. Very fine particles, 63 μm (230 mesh) may be limited to 15% or less except for carbon or MgO-containing materials that are to be a component of a pelletized product. Materials used for pellet making have no practical limit on particle size since fine particles will be agglomerated with binders in the mixing process.

When a carbonaceous material is included in the slag conditioner, the slag conditioner may comprise at least 10 wt. % of the carbonate-containing material and up to 40 wt. % of the carbonate-containing material, for example, 10-40 wt. % of the carbonate-containing material, 15-35 wt. % of the carbonate-containing material, or 15-30 wt. % of the carbonate-containing material.

The carbonate-containing material and the carbonaceous material are contained in amounts such that the carbonate to carbon (CO₃:C) weight ratio of the slag conditioner is at least 0.1 and up to 5, for example, 0.1-5, 0.1-3, or 0.1-2.

A first example of the slag conditioner comprises 30 wt. % dolomite which contains 65 wt. % carbonate, 7 wt. % silicon carbide which contains 70 wt. % reducing element (Si), and 63 wt. % carbonaceous material which contains 90 wt. % carbon. Thus, the slag conditioner contains 19.5 wt. % carbonate, 4.9 wt. % reducing element (Si), and 56.7 wt. % carbon for a CO₃:reducing element ratio of 19.5:4.9 or 4.0 and a CO₃:carbon ratio of 19.5:56.7 or 0.3.

A second example of the slag conditioner comprises 30 wt. % dolomite which contains 65 wt. % carbonate, 5 wt. % silicon carbide which contains 70 wt. % reducing element (Si), and 65 wt. % carbonaceous material which contains 90 wt. % carbon. Thus, the slag conditioner contains 19.5 wt. % carbonate, 3.5 wt. % reducing element (Si), and 58.5 wt. % carbon for a CO₃:reducing element ratio of 19.5:3.5 or 5.6 and a CO₃:carbon ratio of 19.5:58.5 or 0.3.

A third example of the slag conditioner comprises 30 wt. % magnesite which contains 71 wt. % carbonate, 7 wt. % silicon carbide which contains 70 wt. % reducing element (Si), and 63 wt. % carbonaceous material which contains 90 wt. % carbon. Thus, the slag conditioner contains 21.3 wt. % carbonate, 4.9 wt. % reducing element (Si), and 56.7 wt. % carbon for a CO₃:reducing element ratio of 21.3:4.9 or 4.3 and a CO₃:carbon ratio of 21.3:56.7 or 0.4.

A fourth example of the slag conditioner comprises 30 wt. % magnesite which contains 71 wt. % carbonate, 5 wt. % silicon carbide which contains 70 wt. % reducing element (Si), and 65 wt. % carbonaceous material which contains 90 wt. % carbon. Thus, the slag conditioner contains 21.3 wt. % carbonate, 3.5 wt. % reducing element (Si), and 58.5 wt. % carbon for a CO₃:reducing element ratio of 21.3:3.5 or 6.1 and a CO₃:carbon ratio of 21.3:58.5 or 0.4.

This slag conditioner may be best suited for carbon steel production, stainless steel production, and alloy steel production.

Slag Conditioner with Carbonate-Containing Material, a Reducing Agent, a Carbonaceous Material, and an MgO-Containing Material

The slag condition described above which contains a carbonate-containing material, a reducing agent, and a carbonaceous material may also contain an MgO-containing material.

The slag conditioner may comprise at least 5 wt. % of an MgO-containing material and up to 25 wt. % of a MgO-containing material, for example, 5-25 wt. % of an MgO-containing material, 5-20 wt. % of an MgO-containing material, or 10-20 wt. % of an MgO-containing material.

The carbonate-containing material and the MgO-containing material are contained in amounts such that the carbonate to MgO (CO₃:MgO) weight ratio of the slag conditioner is at least 0.1 and up to 10, for example, 0.1-10, 0.1-5, or 0.5-3.

The carbonate-containing material and the carbonaceous material are contained in amounts such that the carbonate to carbon (CO₃:C) weight ratio of the slag conditioner is at least 0.1 and up to 5, for example, 0.1-5, 0.1-3, or 0.1-2.

A first example of the slag conditioner comprises 22 wt. % dolomite which contains 65 wt. % carbonate, 5 wt. % silicon carbide which contains 70 wt. % reducing element (Si), 58 wt. % carbonaceous material which contains 90 wt. % carbon, and 15 wt. % MgO-containing material containing 80 wt. % MgO. Thus, the slag conditioner contains 14.3 wt. % carbonate, 3.5 wt. % reducing element (Si), 52.2 wt. % carbon, and 12.0 wt. % MgO for a CO₃:reducing element ratio of 14.3:3.5 or 4.1, a CO₃:carbon ratio of 14.3:52.2 or 0.3, and a CO₃:MgO ratio of 14.3:12.0 or 1.2.

A second example of the slag conditioner comprises 24 wt. % dolomite which contains 65 wt. % carbonate, 4 wt. % silicon carbide which contains 70 wt. % reducing element (Si), 60 wt. % carbonaceous material which contains 90 wt. % carbon, and 12 wt. wt. % MgO-containing material containing 80 wt. % MgO. Thus, the slag conditioner contains 15.6 wt. % carbonate, 2.8 wt. % reducing element (Si), 54.0 wt. % carbon, and 9.6 wt. % MgO for a CO₃:reducing element ratio of 15.6:2.8 or 5.6, a CO₃:carbon ratio of 15.2:54.0 or 0.3, and a CO₃:MgO ratio of 15.2:9.6 or 1.6.

A third example of the slag conditioner comprises 22 wt. % magnesite which contains 71 wt. % carbonate, 5 wt. % silicon carbide which contains 70 wt. % reducing element (Si), 58 wt. % carbonaceous material which contains 90 wt. % carbon, and 15 wt. % MgO-containing material containing 80 wt. % MgO. Thus, the slag conditioner contains 15.6 wt. % carbonate, 3.5 wt. % reducing element (Si), 52.2 wt. % carbon, and 12.0 wt. % MgO for a CO₃:reducing element ratio of 15.6:3.5 or 4.5, a CO₃:carbon ratio of 15.6:52.2 or 0.3, and a CO₃:MgO ratio of 15.6:12.0 or 1.3.

A fourth example of the slag conditioner comprises 24 wt. % magnesite which contains 71 wt. % carbonate, 4 wt. % silicon carbide which contains 70 wt. % reducing element (Si), 60 wt. % carbonaceous material which contains 90 wt. % carbon, and 12 wt. % MgO-containing material containing 80 wt. % MgO. Thus, the slag conditioner contains 17.0 wt. % carbonate, 2.8 wt. % reducing element (Si), 54.0 wt. % carbon, and 9.6 wt. % MgO for a CO₃:reducing element ratio of 17.0:2.8 or 6.1, a CO₃:carbon ratio of 17.0:54.0 or 0.3, and a CO₃:MgO ratio of 17.0:9.6 or 1.8.

This slag conditioner may be best suited for carbon steel production, stainless steel production, and alloy steel production.

Additional Constituents

Iron or iron oxide and other compatible fillers up to 25 wt. % may be added depending on the desired effects on the slag. Iron oxide may be added when carbon steels are being melted. The iron oxide reacts with carbon injected into the slag to produce carbon dioxide or carbon monoxide gas in order to foam the slag. However, the addition of iron oxide to stainless steels or other steels containing valuable alloying elements such as chromium does not have the same effect. In these cases, the iron oxide is reduced by the valuable alloying elements reducing the yield of those elements in the steel.

In one aspect of the invention, the CO₃-containing material particles and the reducing agent particles, along with any optional additives may be mixed, and the resulting slag conditioner may be introduced directly into the slag in powder or aggregate form. In this case, the slag conditioner may contains no more than 5 wt. % moisture, for example, no more than 2 wt. % moisture.

In another aspect of the invention, the slag conditioner may be pelletized for introduction into the slag. In order to pelletize the slag conditioner, at least 1 wt. % of a binder and not more than 14 wt. % of a binder, for example, 1-14 wt. % of a binder or 5-14 wt. % of a binder is added to the CO₃-containing material and reducing agent. The binder may be one or more material selected from the group including, but not limited to, sodium silicate, calcium hydroxide, ligosulfonate, lignosulfonate solutions, hydrochloric acid, sulfuric acid, magnesium chloride, magnesium sulfate, molasses, pitch, tar, asphalt, bentonite, clay, starch, and resin.

The CO₃-containing material, the reducing agent, and the binder are blended in any suitable mixer having an impeller or mixing blades, for example, an Eirich mixer, a Day mixer, a barrel mixer, or a ribbon mixer. During mixing, the material agglomerates forming individual pellets. Such a process is often referred to as a granulation process. The resulting pellets may be screened to produce a final slag conditioner having pellets that are at most 13 mm, for example, at most 7 mm, and at least 0.25 mm, for example, at least 0.5 mm. At least 85% of the particles may be at least 0.25 mm, for example, at least 0.5 mm. The pellets are dried so that they contain less than 5 wt. % moisture, for example, less than 2 wt. % moisture.

In another aspect of the invention, the CO₃-containing material, the reducing agent, and the binder may be agglomerated and briquetted or extruded to form larger briquettes or lumps that can be directly charged into the top of the furnace. For example, the briquettes may be 5-8 cm by 1-2 cm.

In another aspect of the invention, the briquettes or lumps may be crushed to form pellets of the size previously described.

In another aspect of the invention, the CO₃-containing material and the reducing agent may be directly charged into the top of the furnace. The CO₃-containing material and reducing agent may comprise particles that are, for example, 8 mm or less.

Upon introduction of the slag conditioner into the slag in the EAF, the carbonate is heated to form carbon monoxide (CO) and carbon dioxide (CO₂) gases which foam the slag. The foamy slag protects the refractory lining of the furnace such that the life of the furnace, i.e., the operation time before the refractory lining must be replaced, is increased and less maintenance materials are needed. The energy that is lost due to the heating of the CO₃-containing material to form the carbon monoxide and carbon dioxide gases is replaced by exothermic oxidation of the reducing element contained in the reducing agent. The reaction that oxidizes the reducing element at the same time reduces oxides present in the slag including oxides of valuable alloying elements, thereby increasing the alloying elements that are provided to the steel and thus the alloy yield.

When the slag conditioner contains the optional MgO-containing material, the slag viscosity is increased via the MgO as periclase, thereby creating a creamy slag that coats the refractory linings of the EAF walls, which also contributes to increasing the life of the furnace. In this way, during the production of stainless steel, the slag conditioner provides a foamy slag with high bulk “effective” viscosity while avoiding oxidation of valuable elements, such as chromium and nickel and reducing oxides of the same valuable elements without requiring any additional energy input. The foamy slag with high bulk or “effective” viscosity protects the refractory lining of the furnace, thereby increasing the operation time before the refractory lining must be replaced.

When the slag conditioner contains the optional carbonaceous material, the carbon reduces iron oxide and other oxides present in the slag to create carbon dioxide that foams. In the production of carbon and alloy steels, where oxygen may be injected into the furnace to provided supplemental heating, thereby resulting in the formation of substantial iron oxide, the carbon addition is especially beneficial.

Whereas particular aspects of this invention have been described above for purposes of illustration, it will be evident to those skilled in the art that numerous variations of the details of the present invention may be made without departing from the invention as defined in the appended claims. 

The invention claimed is:
 1. A slag conditioner for electric arc furnace steel production comprising: 50-90 wt. % of a carbonate-containing material with the balance being a reducing agent that comprises a reducing element that is easily oxidized in an exothermic reaction, wherein the weight ratio of CO₃ to the reducing element (CO₃:reducing element) for the slag conditioner is 3-20.
 2. The slag conditioner of claim 1, wherein the carbonate-containing material is one or more material selected from the group consisting of dolomite and limestone.
 3. The slag conditioner of claim 1, wherein the reducing element is selected from the group consisting of silicon and aluminum.
 4. The slag conditioner of claim 1, further comprising 5-25 wt. % of an MgO-containing material, wherein the CO₃ to MgO (CO₃:MgO) weight ratio for the slag conditioner is 1-15.
 5. The slag conditioner of claim 4, wherein at least 50% of the MgO in the MgO-containing material is periclase.
 6. The slag conditioner of claim 1, wherein the slag conditioner is a particulate comprising particles of the carbonate-containing material mixed with particles of the reducing agent or is a pellet comprising particles of the carbonate-containing material mixed with particles of the reducing agent.
 7. The slag conditioner of claim 1, wherein the slag conditioner is a briquette.
 8. A method of conditioning the slag in an electric arc furnace where steel is being produced, the method comprising introducing the slag conditioner of claim 6 into the slag or into an interface between the slag and the molten metal.
 9. A method of conditioning the slag in an electric arc furnace where steel is being produced, the method comprising charging the slag conditioner of claim 7 into the top of the furnace.
 10. A slag conditioner for electric arc furnace steel production comprising a carbonate-containing material, a carbonaceous material, and a reducing agent that comprises a reducing element that is easily oxidized in an exothermic reaction, wherein the slag conditioner comprises 10-40 wt. % of the carbonate-containing material, 8-87 wt. % of the carbonaceous material, and the balance is the reducing agent, and the weight ratio of CO₃ to the reducing element (CO₃:reducing element) is 3-20 and the weight ratio of CO₃ to the carbon provided by the carbonaceous material (CO₃:C) is 0.1-5.
 11. The slag conditioner of claim 10, wherein the carbonate-containing material is one or more material selected from the group consisting of dolomite and limestone.
 12. The slag conditioner of claim 10, wherein the reducing element is selected from the group consisting of silicon and aluminum.
 13. The slag conditioner of claim 10, wherein the carbonaceous material is one or more material selected from the group consisting of anthracite coal, semi-anthracite coal, bituminous coal, natural graphite, synthetic graphite, petroleum coke, metallurgical coke, spent EAF electrodes, spent carbon anodes, and carbon black.
 14. The slag conditioner of claim 10, further comprising 5-25 wt. % of an MgO-containing material, wherein the CO₃ to MgO (CO₃:MgO) weight ratio for the slag conditioner is 0.1-10.
 15. The slag conditioner of claim 14, wherein at least 50% of the MgO in the MgO-containing material is periclase.
 16. The slag conditioner of claim 10, wherein the slag conditioner is a particulate comprising particles of the carbonate-containing material, the carbonaceous material, and the reducing agent or is a pellet comprising particles of the carbonate-containing material, the carbonaceous material, and the reducing agent.
 17. The slag conditioner of claim 10, wherein the slag conditioner is a briquette.
 18. A method of conditioning the slag in an electric arc furnace where steel is being produced, the method comprising introducing the slag conditioner of claim 16 into the slag or into an interface between the slag and the molten metal.
 19. A method of conditioning the slag in an electric arc furnace where steel is being produced, the method comprising charging the slag conditioner of claim 17 into the top of the furnace. 